I don't use phpBB* at all, so I can't tell.... the question is then, if that's a problem of open_basedir or phpBB....

But you are right, open_basedir restrictions can sometimes take you in trouble, e.g. when using pear's AJAX or similar things. But then you just have to correct your open_basedir and safemode's openbasedir thing for your specific vhost, or globally. (in case of adminCPs like ISPCOnfig it's not that easy, but possbile for general changes :-\)